In modern gas turbine engines, the high pressure turbine section hardware, such as turbine blades and vanes, is exposed to temperatures in excess of 1000 degrees C. for extended periods of time. At such temperatures, high temperature oxidation is the most important form of environmental attack observed with aluminide diffusion coatings. High temperature oxidation is a chemical reaction whose rate controlling process for an aluminide coating is diffusion through a product (oxide) layer. Diffusion is a thermally activated process, and consequently, the diffusion coefficients are exponential functions of temperature. Since the oxidation of aluminide coatings is a diffusion controlled reaction and diffusion coefficients are exponential functions of temperature, the oxidation rate is also an exponential function of temperature. At low temperatures where diffusion coefficients are relatively small, the growth rate of a protective scale on any aluminide coating is also small. Thus, adequate oxidation resistance should be provided by any state of the art aluminide coatings, such as: chromium aluminide, aluminide or two phase [PtAl.sub.2 +(Ni,Pt)Al] platinum aluminide, all inward grown coatings made by pack cementation. However, at high temperatures where the diffusion coefficients and consequently the oxidation rate increase rapidly with increasing temperature, only coatings which form high purity alumina (Al.sub.2 O.sub.3) scales are likely to provide adequate resistance to environmental degradation.
The presence of platinum in nickel aluminide has been concluded to provide a number of thermodynamic and kinetic effects which promote the formation of a slow growing, high purity protective alumina scale. Consequently, the high temperature oxidation resistance of platinum modified aluminide diffusion coatings generally is better as compared to aluminide diffusion coatings not containing platinum.
In recent years, several limitations of the industry standard, two phase [PtAl.sub.2 +(Ni,Pt)Al], inward grown platinum aluminide coatings have been identified. First, the two phase coatings have metastable phase assemblages and thicknesses, as demonstrated in engine tests at both General Electric and Rolls-Royce. Second, the two phase coatings are sensitive to thermal mechanical fatigue (TMF) cracking in engine service, and the growth of these coatings in service only makes this problem worse. Third, the thick, inward grown platinum aluminides exhibit rumpling during both cyclic oxidation and engine testing. This phenomenon can have particularly undesirable consequences when platinum aluminide coatings are used as the bond coat in thermal barrier coating systems. Fourth, the two phase platinum aluminide coatings are hard and brittle, and this can result in chipping problems during post coat handling and assembly operations.
Many of the problems encountered with the previous industry standard platinum aluminides can be attributed to the two phase, inward grown structure and can be overcome by using outwardly grown, single phase platinum aluminde coatings as described, for example, in the Conner et al. technical articles entitled "Evaluation of Simple Aluminide and Platinum Modified Aluminide Coatings on High Pressure Turbine Blades after Factory Engine testing", Proc. AMSE Int. Conf. of Gas Turbines and Aero Engine Congress Jun. 3-6, 1991 and Jun. 1-4, 1992. For example, the outwardly grown, single phase coating microstructure on directionally solidified (DS) Hf-bearing nickel base superalloy substrates was relatively unchanged after factory engine service in contrast to the microstructure of the previous industry standard two phase coatings. Further, the growth of a CVD single phase platinum aluminide coating was relatively insignificant compared to two phase coatings during factory engine service. Moreover, the "high temperature low activity"outward grown platinum aluminde coatings were observed to be more ductile than inward grown "low temperature high activity" platinum aluminide coatings.
In the production of platinum modified aluminide diffusion coated gas turbine engine components, such as blades and vanes, the components are conventionally electroplated to deposit platinum metal on their gas path surfaces prior to aluminizing. Some plating baths used in the past employ hexachloro platinic acid ((H.sub.2 PtCl.sub.6) as a source of platinum with examples including the phosphate buffer solution as described in U.S. Pat. Nos. 3,677,789 and 3, 819,338 or an acid chloride bath similar to that outlined by Atkinson in Trans. Inst. Metal Finish. vol. 36 (1958 and 1959) page 7. Sulfate solutions also have been used in the past which utilize a P salt [(NH.sub.3).sub.2 Pt(NO.sub.2).sub.2 ] precursor as described by Cramer et al. in Plating vol. 56 (1969) page 516 or H.sub.2 Pt(NO.sub.2).sub.2 SO.sub.4 precursor as described by Hopkins et al. in Plat. Met. Rev. vol. 4 (1960) page 56. Finally, some platinum aluminide coating procedures utilize a platinum Q salt [(NH.sub.3).sub.4 Pt(HPO.sub.4)] bath as discussed by Albon, Davis, Skinner and Warren in U.S. Pat. No. 5,102,509. Conventionally well known platinum plating baths contain high concentrations of sulfur and/or phosphorous and/or chlorine, and the deposition reactions in all these baths involve complex ions with ligands containing sulfur and/or phosphorous and/or chlorine.